Heat shrinkable films have been extensively used, e.g., for labeling bottles, batteries or electrolytic condensers, and wrapping containers and other products. Such heat-shrinkable films are formed of polyvinyl chloride, polystyrene or polyester, and they are required to have good sealing and shrinking properties as well as good heat resistance, chemical resistance, weatherability and printability.
Conventional heat-shrinkable films formed of polyvinyl chloride or polystyrene have relatively poor heat resistance, chemical resistance, weather resistance and heat-shrinking properties. In particular, polyvinyl chloride-based heat-shrinkable films recently become increasingly disfavored because they emit toxic pollutants when incinerated. Polystyrene films, on the other hand, have the problem of poor printability, requiring a special purpose ink. Polystyrene films also tend to undergo spontaneous shrinkage during long-term storage.
Heat-shrinkable polyester films formed of polyethylene terephthalate (PET) have satisfactory heat resistance, chemical resistance, weatherability and shrinking properties. However, the shrinkage stress and shrinkage ratio of a PET film are generally unacceptably high, giving non-uniform shrink-wrappings with consequential distortion of images printed thereon.
Japanese Laid-open Patent Publication Nos. 63-139725, 7-53416, 7-53737, 7-216107, 7-216109 and 9-254257 disclose that the shrinkage uniformity of a PET film can be improved by blending a polyethylene terephthalate with a polybutylene terephthalate in a particular ratio, or by copolymerizing a dicarboxylic acid component such as terephthalic acid and isophthalic acid with a diol component such as ethylene glycol and 1,4-cyclohexanedimethanol. Although this heat-shrinkable film shows some improvement in terms of uniform shrinkage, it is difficult in handle due to its tendency to gather static electricity, unacceptably low Tg and poor slipping property.